An optical time domain reflectometry (OTDR) technique can be used to measure values of a physical parameter of interest along an optical fiber. In one application, the optical fiber can be deployed in a wellbore that is used to produce fluids from a reservoir in a subterranean structure, where the reservoir can include hydrocarbons, fresh water, and so forth. Examples of physical parameters that may be of interest include temperature, strain, and other parameters. One type of OTDR technique is the Brillouin OTDR technique, which involves measurements based on Brillouin scattering.
In an optical fiber, Brillouin scattering is an inelastic phenomenon that results from the interaction of incident optical photons (of an incident optical signal) with acoustic phonons in the medium (the optical fiber). This interaction induces a counter-propagating optical wave (reflected or backscattered optical signal) having a frequency (Brillouin frequency) that is shifted from the frequency of the original incident optical wave. Brillouin scattering in an optical fiber is sensitive to both temperature and strain changes in the optical fiber.
A Brillouin backscattered optical signal can have two different components: a Stokes line and an anti-Stokes line. The Stokes and anti-Stokes lines are components of the backscattered optical signal centered on different wavelengths (where the Stokes line is centered on a longer wavelength than the anti-Stokes line). Many conventional Brillouin reflectometer configurations provide only one of the Stokes and anti-Stokes lines, or provide both the Stokes and anti-Stokes lines at the same frequency. In the former case, limited information is available. In the latter case, the contribution from each of the Brillouin Stokes and anti-Stokes lines cannot be assessed individually, which may limit flexibility and accuracy.